President Donald Trump was expected to start signing directives on Wednesday to begin building a wall along the U.S. border with Mexico and to boost the numbers of agents policing illegal immigration, moving quickly on sweeping plans to curb immigration and boost national security.

Trump, who took office last Friday, will begin signing the orders at the Department of Homeland Security, whose responsibilities include immigration and border security, congressional aides with knowledge of the plan said.

In the coming days, the Republican president is also expected to take steps to curb legal immigration including executive orders restricting refugees and blocking the issuing of visas to people from several Muslim-majority Middle Eastern and North African countries including Syria, the sources said.

On Twitter on Tuesday night, Trump reiterated his promise to build a wall along the roughly 2,000-mile (3,200-kilometer) U.S.-Mexico border.

Trump made cracking down on illegal immigration a key element of his presidential campaign, with supporters often chanting "build the wall" during his rallies. Trump has long said that he will make Mexico pay for the wall, but Mexican officials have forcefully resisted this idea.

The cost and nature of the wall have not been made clear. Many Democrats have opposed the plan and could try to thwart any legislation to pay for the construction in the U.S. Congress, although Republicans control both the Senate and House of Representatives.

The border enforcement order includes plans to hire 5,000 more U.S. Customs and Border Protection agents used to apprehend people seeking to slip across the border and to triple the number of U.S. Immigration and Customs Enforcement agents used to arrest and deport immigrants living in the United States illegally.

The Customs and Border Protection agency has struggled to meet its hiring mandate, with a little more than 19,000 agents on the payroll, out of a congressionally mandated 21,000.

Trump's actions could fundamentally change the American stance on immigration, as well as further testing relations with Mexico.

Many Americans view their country with pride as "a nation of immigrants," and President John Kennedy wrote a book with that title more than half a century ago. But Trump successfully tapped into resentment toward the roughly 11 million illegal immigrants already in the United States and said during the campaign he would deport them all.

Trump, who in announcing his presidential bid in June 2015 accused Mexico of sending rapists and criminals into the United States, has also threatened to slap hefty taxes on companies that produce in Mexico for the U.S. market and to tear up the North American Free Trade Agreement (NAFTA) between the Mexico, Canada and the United States.

Trump and Mexican President Enrique Peña Nieto are due to meet next week.

'EASILY BREACHED'

Asked about Trump's wall, U.S. Republican Senator John McCain said a physical barrier is not enough to secure the border and called for the additional use of observation towers, drones and other technology.

Immigration enforcement away from the border is also expected to be strengthened by seeking an end to "sanctuary cities," often governed by Democrats, where local officials refuse to cooperate with federal authorities on actions against illegal immigrants.

Trump will call for an end to this practice and may instruct the federal government to stop providing certain funds to cities that refuse to comply.

Later in the week, Trump is expected to suspend the issuing of visas to people from countries where it is deemed that adequate screening cannot occur. Immigration experts expect those countries to include Syria, Sudan, Somalia, Iraq, Iran, Libya and Yemen.

Trump is expected to limit the number of refugees admitted to the United States to 50,000 a year, down from 100,000, and to impose a temporary ban on most refugees.

A review will be conducted by the Trump administration to determine what screening must occur before travel for citizens from such countries can resume.

During the campaign, Trump proposed barring non-U.S. citizen Muslims from entering the United States, which he said would protect Americans from attacks by Islamist militants like those targeting European cities.

The proposal prompted a furor at home and abroad. His expected directive would instead focus the restrictions on countries of origin rather than explicitly in religion. Still, critics say that the measure contradicts the American spirit.

"To use world events as an excuse to keep people from coming to the United States who are literally fleeing for their lives disrespects the history of this country. It disrespects the many people that came here and made this country what it is," said David Leopold, an immigration attorney and former president of the American Immigration Lawyers Association.

Separately, Trump continues to insist that millions of illegal immigrants voted unlawfully in the Nov. 8 election. He wrote on Twitter on Wednesday he would seek a "major investigation" on alleged voter fraud in the election, despite an overwhelming consensus among state officials, election experts and politicians that such fraud is rare in the United States.

Trump won the state-by-state Electoral College tally that decides the presidency, but he lost the popular vote to Democrat Hillary Clinton by nearly 3 million votes.